This invention concerns a process for treating tubers or roots, and more generally, vegetables and fruit such as potato, beet, turnip, carrot, salsify etc., before cooking.
The invention applies particularly, but not exclusively, to the processing of potato tubers in the production of French fries.
Traditionally, the processing of vegetables includes a sequence of automated stages, during which vegetable tubers or roots are washed, peeled, size-sorted, cut, blanched, dried, possibly immersed in frying oils, frozen and packed. In the cutting stage, the vegetables are sliced into pieces, rings, or specifically as relates to French fries, into strips.
By way of example, the description below will particularity focus on the processing of potato tubers intended to make French fries, which is a preferred application of the process of this invention.
In order to ease tuber cutting, in other words, to reduce resistance to cutting, tubers traditionally are submitted to heat processing, which increases their flexibility. Typically, the tubers are directed toward a horizontal cylindrical tank, which is equipped with an endless screw, and contains water heated to a temperature of 40xc2x0 to 60xc2x0 C., where they are left for a period of 20 to 40 minutes. This process of preparing the tuber for cutting through heat processing, requires a major volume of water (from 25 to 30 cubic meters of water for an automated line producing 15 tonnes of finished product per hour); this volume of water also has to be renewed, heated and maintained at the appropriate temperature.
This typical process has a number of drawbacks, including:
At the surface of the tuber, a cooking ring is produced, due to starch gelatinization. This degrades the quality of the finished product in terms of texture and colour. The colour difference of the cook ring is viewed as a defect to be eliminated, which results in a loss of material;
Losses resulting from dissolution of tuber material in the water;
Odors that may develop due to bacterial activity in the water and biological reactions;
Processing which is not homogenous, since it operates through diffusion toward the inside of the tubers; a desired inside temperature is reached more rapidly in small tubers than in medium or large tubers.
Pre-treatment to reduce the tubers"" resistance to cutting is necessary to ease the action of cutting tools, avoid twisting of the cut strips or a poor cut, as well as damage to the cutting knives. Inappropriate pre-treatment may result in a non-shear cut, and a xe2x80x9cshatteringxe2x80x9d or crushing effect, that appears on the sliced strips as slits that are likely to make the strips fragile. Non-shear cutting means a major deterioration of the vegetable cells in the product, and therefore, large loss of material due to deteriorated cells. If a strip breaks below a minimal desired length, lit will have to be eliminated or directed to a stream for by-product processing. Also, slits that appear during non-shear cutting will take on a brown discoloration during the cooking process; that harms the product appearance and increases oil absorption.
It would be desirable to submit tubers to treatment that will prepare them for cutting, and is efficient; in other words, a kind of treatment which avoids the xe2x80x9cshatteringxe2x80x9d effect, while minimizing or even eliminating the drawbacks that were mentioned about traditional preheating.
To overcome the limitations of conventional methods of pretreating tubers, solutions involving electrical processes were investigated.
The application of pulsed electric fields is known in various areas of food product processing, for example food industry, specifically for sugar extraction from beets, or sterilization of food products.
U.S. Pat. No. 3,997,678 (Vigerstrom) discloses a process for processing potato tubers that are intended to be made into French fries. The process includes a blanching stage between the cutting and the immersion into frying oil, during which the strips are immersed in a bath of water and heated until sterilization is achieved by application of an electric field through electrodes immersed in the water. Traditionally, a blanching or precooking stage is included in processes for making tubers into fries. It is meant to reduce the temperature or the frying time, and consists in a preheating stage, in order to extract reducing sugars and inactivate enzymes. It is seen as a gelatinization of starch.
In contrast to the process that is disclosed in U.S. Pat. No. 3,997,678, in which the application of an electric field occurs following cutting to heat and sterilize the product, the present invention focuses on treatment before cutting, to reduce the tuber resistance to cutting.
The invention provides a process that includes, as well known, a treatment stage for tubers or roots, and more generally, of vegetables and fruit in order to reduce their resistance to cutting, and thus reduce any loss of material during subsequent stages of the manufacturing process.
According to the invention, said stage consists in the application of a high electric field directly to vegetables and fruit, under such conditions that the resulting temperature increase for the vegetables and fruit is almost zero or at least sufficiently low as not to amount to a preheating stage. The application of a high electric field, such as is used for extracting sugar from beet and precooking fries, translates to vegetables and fruit, and particularly to potato tubers, with the effect of softening which is favourable to shear cutting during subsequent stages for transforming the tubers into fry strips. The process is found to be insensitive to tuber volume, whatever the size of the tuber, with the absence of any noticeable elevation of the tuber temperature; as such no cooking ring is formed that would lead to loss of material as occurs in the case of heat processing.
Tests have shown that, to obtain optimal cutting, tubers should be immersed in water and an electric field of 46 to 65 V/cm applied between electrodes in the water, during a period of between 3 and 5 seconds.